Broadband wireless is expected to be one of the main drivers of the telecommunications industry. There is a substantial increase in demand for broadband wireless connectivity, with wireless broadband being the key growth engine for mobile wireless broadband networks.
The traditional approach for mobile network infrastructure deployment is similar to that of cellular phone networks. The network is based on macro-cell deployment, that is, the base stations, radios and antennas are installed on top of high towers, transmitting at high power, so as to maximize the base station coverage area. However, as smaller and smaller cells are utilized, the increasing numbers of base stations, particularly femtocells, in a geographical area create a major problem of interference management, due to interference between base stations.
In conventional wireless telecommunications networks, mobile stations must synchronize to a nearby base station. In addition, in order for the mobile wireless network to operate properly, synchronization is required between the different base stations. The quality of the synchronization in the network directly impacts the interference generated by the network. The transmission of voice, video and data through any communication network requires a stable frequency reference, and precise frequency synchronization is especially critical in mobile networks for successful call signal hand-off between base stations, as well as for the transport of real-time services.
Femto- and pico-cells can be connected to a conventional wired backhaul which is IP-based, for example, DSL (XDSL) or other Ethernet-based wire line communication techniques. One technique for providing synchronization to the base station connected to this type of backhaul is defined in IEEE Standard 1588, which can provide 1-PPS synchronization over Ethernet. However, IEEE 1588 is limited in time jitter performance, which may violate regulation requirements.
As more networks transition to an IP-centric backhaul, the changes in the backhaul also impact how the network derives an accurate sync feed. In addition, as the size of cells diminishes from macro to micro to pico to femtocells, the need for, and the difficulty of providing, accurate synchronization increase dramatically. For example, GSM network (Global System for Mobile communications) base stations have traditionally derived their long-term frequency accuracy from locking a relatively low-performance quartz oscillator embedded in the base station to a recovered clock signal from a T1/E1 leased line backhaul facility. Timing signals based on a primary reference source (PRS) transmitted over the backhaul keep the embedded oscillator calibrated to within sufficient accuracy. Without a well-synchronized backhaul feed to lock to, the oscillator frequency would drift out of specification in a matter of months or days. Thus, a reliable and accurate time source is required for accurate synchronization.
One solution to this problem is achieved by installing an external GPS receiver and antenna in each base station to provide the external 1-PPS signal to the base station equipment. From this signal, the base station derives and locks the start of the frame and the frequency synchronization. It will be appreciated that GPS satellites transmit the clock time to each base station in the network at the same time.
In indoor network applications, there is great technical difficulty in providing GPS synchronization for each and every base station, because of the low amplitude and wall penetration losses of the GPS transmission, making GPS reception indoors, especially on lower levels, almost impossible. Thus, conventional femtocells cannot be synchronized by GPS, since there is insufficient indoor coverage by the GPS network.
Even if there were GPS coverage, the GPS module cost is too high for use in femtocell home base station applications. Deployment of a GPS antenna for every base station in a micro-/pico-/femtocell deployment would be very expensive, due to equipment and GPS service provider costs.
Accordingly, there is a long felt need for a relatively low cost method of providing accurate and robust synchronization in femtocells in both outdoor and indoor locations.